Improve ErrorModel documentation

Add safir.models to the documentation generation and add an example
of how to use the ErrorModel.

docs/api.rst

@@ -17,5 +17,8 @@ API reference
 .. automodapi:: safir.metadata
    :include-all-objects:
 
+.. automodapi:: safir.models
+   :include-all-objects:
+
 .. automodapi:: safir.middleware.x_forwarded
    :include-all-objects:

src/safir/models.py

@@ -1,15 +1,41 @@
 """Standard models for FastAPI applications.
 
+Examples
+--------
+To reference the `ErrorModel` model when returning an error message, use code
+similar to this:
+
+.. code-block:: python
+
+   @router.get(
+       "/route/{foo}",
+       ...,
+       responses={404: {"description": "Not found", "model": ErrorModel}},
+   )
+   async def route(foo: str) -> None:
+       ...
+       raise HTTPException(
+           status_code=status.HTTP_404_NOT_FOUND,
+           detail=[
+               {"loc": ["path", "foo"], "msg": msg, "type": "invalid_foo"},
+           ],
+       )
+
+Notes
+-----
 FastAPI does not appear to export its error response model in a usable form,
 so define a copy of it so that we can reference it in API definitions to
 generate good documentation.
 """
 
+# Examples and notes kept in the module docstring because they're not
+# appropriate for the API documentation generated for a service.
+
 from typing import List, Optional
 
 from pydantic import BaseModel, Field
 
-__all__ = ["ErrorModel"]
+__all__ = ["ErrorDetail", "ErrorModel"]
 
 
 class ErrorDetail(BaseModel):